The present invention relates to a defect detection system and more particularly to a defect detection system for detecting defects in minute patterns.
A defect detection system for checking any defect in wiring patterns or the like on a semiconductor substrate compares a reference pattern free from any defect with a pattern to be checked to detect a point with different pattern shapes as a defect.
FIGS. 7A and 7B show an example of a defect detection process described in JP, 5-281154, A. FIG. 7A is a top view of a pattern to be checked or a reference pattern 50. The central rectangular region in a form of 4xc3x974 matrix is a region to be checked 51. The regions along the four sides are alignment regions 52.
In such a checking process, an edge detection filter 53 composed of successive four CCD pixels shown in FIG. 7B is used. For example, a case where an edge to be detected extends vertically is considered. The edge detection filter 53 is overlaid on the alignment region 52 above the region to be checked 51. Then, gray levels of four pixels (fi,j, fi+1,j, fi+2,j, fi+3,j) of the edge detection filter 53 are detected. The filter output of the four pixels is shown by the following equation:
fi,jxe2x88x92fi+1,jxe2x88x92fi+2,j,; fi+3, j.
As shown in FIG. 7B, the following detection results are obtained: (1) when the filter output is negative (xe2x88x92), the edge is between fi, j and fi+1, j; (2) when the filter output is 0, the edge is between fi+1, j and fi+2, j; and (3) when the filter output is positive (+), the edge is between fi+2, j and fi+3, j. In this manner, the edge detection is performed to both of the pattern to be checked and the reference pattern at the four peripheral sides of the region to be checked 51.
Next, each difference of the edge positions at the four sides between the pattern to be checked and the reference pattern is obtained. Then, the relative position of the pattern to be checked and the reference pattern is corrected so that the sum of the absolute values of such differences is minimal.
At last, the pattern to be checked and the reference pattern are compared with each other and, if there is any difference therebetween, it is determined that the checked pattern has a defect.
In such a defect detection process, the relative position between the pattern to be checked and the reference pattern is corrected in one pixel of the edge detection filter 53 (one pixel of the CCD). Therefore, misalignment less than one pixel cannot be corrected. When a wiring layer accounts for 60% of one pixel of the edge detection filter 53, for example, such misalignment cannot be detected because the wiring layer is considered to occupy whole one pixel.
However, with recent micro miniaturization of semiconductor devices, the pattern width of a wiring layer, etc. becomes equal to or smaller than the width of one CCD pixel. Thus the size of a defect to be detected by a defect detection system is less than one pixel. When there is misalignment less than one pixel between a pattern to be checked and a reference pattern, it is detected as a defect in the pattern although it is not an actual defect, resulting in a problem of a false defect.
Therefore, it is an object of the present invention to provide a defect detection system to correct a misalignment less than one pixel between a pattern to be checked and a reference pattern and prevent occurrence of a false defect.
The present invention provides a defect detection system for checking any defect in a pattern to be checked by comparing the pattern with a reference pattern, including: a stage for carrying a substrate on which a pattern to be check is formed; an edge detection means for detecting a pattern edge of the pattern to be checked by scanning laser beams on the substrate and detecting reflected light thereof; an imaging means positioned above the stage and for picking up an image of the pattern to be checked using a charge coupled device (CCD); a detection means for checking for any defect in the pattern to be checked by comparing data of the pattern to be checked obtained by the imaging means with data of the reference pattern; and an adjustment means for adjusting the relative position of the substrate and the imaging means, in which the imaging means picks up an image of the pattern to be checked after the adjustment means adjusts the relative position of the substrate and the imaging means so that the pattern edge of the pattern to be checked is aligned with a side of pixels of the CCD.
With such a defect detection system, a pattern edge is detected using laser beams and the pattern edge is aligned with the edge of the pixels. This can prevent occurrence of a fault defect resulting from the misalignment of the pattern edge and the edge of the pixels and thus perform precise checking of the pattern.
Especially, high-precision defect detection can be performed even for a pattern with a width smaller than that of one pixel of the imaging camera. For this reason, high-precision defect detection of micro-miniaturized and integrated semiconductor devices can be performed easily.
Preferably, the edge detection means includes a light source of said laser beams and a detector for receiving reflected light of the laser beams.
The use of such laser beams allows accurate detection of the pattern edge of the pattern to be checked.
Preferably, the edge detection means is a means for detecting said pattern edge by radiating the substrate with two independent laser beams from two different directions.
Preferably, the horizontal components (components substantially parallel to the surface of the substrate) of the two laser beams are substantially orthogonal to each other.
Preferably, the edge detection means is a means for detecting a position radiated with the laser beams at peak scattering light intensity of the reflected light as the pattern edge of the pattern to be checked.
Preferably, the incident angle of the laser beams is approx. 45xc2x0 with respect to the substrate on which a pattern to be checked is formed.
The edge detection means may include a polarizer selected between a linear polarizer and phase shifter, polarizes the laser beams and radiates them on the substrate.
The use of polarized laser beams for detection of the pattern edge facilitates the detection of change in scattering intensity in the pattern edge and allows accurate detection of the pattern edge.
Preferably, the adjustment means is a means for adjusting the relative position of the stage and the imaging means by moving at least one of the stage and the imaging means.
In other words, considering the configuration and structure of the defect detection system, it is preferable that the stage and/or imaging means should be moved so that the pattern edge of the pattern to be checked on the stage should be aligned with the edge of the pixels of the imaging means.
The data of the reference pattern may be the data of the non-defective pattern selected from a plurality of the patterns to be checked. Based on the image data of the pattern to be checked, the defects of other patterns to be checked are checked.
The data of the reference pattern may be CAD data of the pattern to be checked. Based on such CAD data, the defects of the pattern to be checked are checked.
The pattern width of the pattern to be checked may be smaller than the width of the pixels.
With a defect detection system in accordance with the present invention, a pattern edge is detected using laser beams and the pattern edge can be aligned with the edge of the pixels even for a pattern to be checked with a pattern width smaller than that of CCD pixels in the imaging means. This allows high-precision defect detection even for such a pattern to be checked with a smaller pattern width.
As described above, the use of the defect detection system in accordance with the present invention can prevent the detection of misalignment between the pattern edge and the edge of the pixels as a false defect.
This allows high-precision defect detection of a pattern having a width smaller than that of one pixel of an imaging camera.